1. Field of the Invention
The present invention relates to a device and to a process for mode-locking a laser, and in particular a laser functioning in pulsed mode.
2. Description of Related Art
A laser cavity consists of an optical gain medium placed inside a resonator delimited by two mirrors oriented in auto-collimation, that is to say face to face. When the gain medium is activated, an optical oscillation is maintained in the cavity, such that the device can emit a light beam characterized by a very high spatial and spectral brightness.
The mode-locking of a laser cavity consists in forcing short light pulses to circulate in said resonator, so as to generate pulses of high peak intensity and with a pulse length typically of less than 100 picoseconds, which may be upto a few femtoseconds depending on the gain medium used.
Lasers which may be distinguished include lasers of the continuous type in which the gain medium is permanently activated, that is to say over time scales of from several seconds to several hours. A continuous mode-locked laser may thus generate short pulses at a repeat rate of the order of a few tens to a few hundreds of megahertz, corresponding to the circulation (to and fro) time of the pulses in the resonator.
This high repeat rate implies that a laser of this type will emit low-energy light pulses. Nevertheless, this type of laser is adequate for many applications which require a high mean optical power but which can make do with low pulse energy, such as the LIDAR technology, or “linear” absorption spectroscopy, photoionization spectroscopy, fluorescence spectroscopy, etc.
Moreover, lasers of pulsed type exist, which are characterized by a very low working cycle of the gain medium (of less than 1/50). This gain medium is activated for a short period, typically of less than one millisecond at a low repeat rate typically of a few tens of hertz. In pulsed mode, the gain medium may be temporarily very highly activated, corresponding to a large storage of optical energy in the gain medium, such that a mode-locked pulsed laser will be able to generate pulses of markedly greater energy than those generated by mode-locked lasers of continuous type. However, the fact, firstly, that the amplification factor of the gain medium is not constant during the transient activation period, and, secondly, that the stabilization of the optical oscillation in the laser cavity is a dynamic process which requires a certain amount of time and may thus be incomplete during the activation time of the gain medium, limits the efficiency of the mode-locking and consequently the brevity and energetic stability of said optical pulses generated.
Pulsed lasers are used in manufacturing processes which require high-energy optical pulses, such as for the ablation of materials, laser cutting and surface treatment, and also for “non-linear” optical spectroscopies such as multi-photon resonant ionization or frequency-sum generation spectroscopy, and also any technique requiring a low repeat rate of the laser (time-resolved measurements).
One way for mode-locking lasers of pulsed type is to insert a cell containing a dye (liquid solvent), optionally combined with an intensity limiter, into the laser cavity. This device has several drawbacks, in particular:                the mobility and inhomogeneity of the solvent circulating in the cell are factors causing energy instability of the emitted pulses;        the chemical or photochemical degradation of said dye makes it necessary for technicians to intervene regularly in order to optimize the mode-locking process.        
Document U.S. Pat. No. 4,914,658 describes a solid-state laser such as a Neodymium-doped Yttrium Aluminium Garnet (Nd:YAG) which is combined with a non-linear crystal and a dichroic mirror in order to create a non-linear optical means for mode-locking the laser. In the simplest embodiment of the device, the non-linear crystal makes it possible to generate a beam at the second harmonic from the fundamental beam amplified by the gain medium. The oscillation in the resonant cavity of the portion of the fundamental beam not converted by the non-linear crystal is negatively discriminated by means of a dichroic mirror which must have a reflection coefficient at the second harmonic frequency which is greater than that at the fundamental frequency.
Adjusting the optical distance between the non-linear crystal and the dichroic mirror makes it possible to obtain a suitable phase shift between the fundamental beam and the beam at the second harmonic, so as to obtain an efficient reconversion of the beam at the second harmonic into a fundamental beam in the non-linear crystal. This phase shift can also be obtained by inserting a transparent plate between the non-linear crystal and the dichroic mirror.
The non-linear optical means serves to increase the quality factor of the laser cavity, that is to say to reduce the energy losses of the laser beam by reflection against the dichroic mirror, when the instantaneous power of the beam at the fundamental frequency generated by the gain medium increases. In other words, the non-linear optical means induces a positive feedback on the quality factor of the laser cavity as a function of the instantaneous power of the beam at the fundamental frequency.
The non-linear optical mode-locking device is also characterized in that the ratio of the beam power at the second harmonic relative to the beam power at the fundamental frequency increases as the power of the beam at the fundamental frequency increases.
Document EP-A-0 951 111 proposes a device and a method for mode-locking a laser, preferably also working in continuous mode, which are based on the principle described in document U.S. Pat. No. 4,914,658. In this case, it is proposed to convert part of the laser beam at the fundamental frequency into a beam at the second harmonic by using a non-linear crystal. The oscillation in the resonant cavity of the part of the fundamental beam not converted in the non-linear crystal is negatively discriminated by means of the combination of a retardation plate and a polarizer. In said document, the gain medium is Nd:vanadate, the non-linear crystal is lithium triborate and the retardation plate has a retardation of λ/4=1064 nm and λ/2=532 nm. The retardation plate is placed between the non-linear crystal and the dichroic mirror, while the polarizer is placed between the gain medium and the non-linear crystal.
It is pointed out in said document that the dichroic mirror, placed behind the non-linear crystal, has a reflection coefficient at the second harmonic frequency which is not greater than the reflection coefficient at the fundamental frequency.
The non-linear optical means described in said document serves to increase the quality factor of the laser cavity, that is to say, to reduce the energy losses of the laser beam by reflection against the polarizer, when the instantaneous power of the beam at the fundamental frequency generated by the gain medium increases. In other words, the non-linear optical means induces a positive feedback on the quality factor of the laser cavity as a function of the instantaneous power of the beam at the fundamental frequency.
The devices described in the two above-mentioned documents U.S. Pat. No. 4,914,658 and EP-A 0 951 111 allow the efficient mode-locking of continuous lasers. For example, pulses as short as ˜10 picoseconds FWHM (full width at half maximum) can be generated when an Nd:YAG gain medium is used. However, these devices do not work properly in the case of pulsed lasers. The shortest pulse widths ever obtained by means of the device as described in document U.S. Pat. No. 4,914,658 are 35 picoseconds FWHM (full width at half maximum) in the case of a pulsed Nd:YAG laser. These poor performance levels result from the fact that the gain factor of the active medium, the energy of the optical pulses and thus the conversion yield for the non-linear crystal used in the non-linear device vary greatly in the course of the activation period of the gain medium, which prevents any stabilization of the optical oscillation in the resonant cavity. Moreover, the small number of to-and-fro cycles within the cavity, and thus of interaction with the non-linear device, produced by the optical pulses during the activation time of the gain medium also limits the efficiency of the mode-locking.